The general objective of the proposed research is an increased understanding of the acquisition and manifestation of expertise by young children. The particular focus will be on expertise in an everyday skill, the categorization of concrete objects. The proposed research also will serve to test and refine the Principal Investigator's theory of early conceptual development. to accomplish these objectives, five aspects of expertise at categorization within particular domains will be considered. The first aspect is the acquisition of basic level categories (e.g., dog, hammer). Acquisition of large numbers of these categories in a particular domain e.g., mammals, carpenter's tools) represents an early step in acquisition of domain-specific expertise. the second aspect is the acquisition of subordinate level categories (e.g., collie, claw hammer). Acquisition of multiple categories at this level within a domain is a further step in the acquisition of expertise. the third aspect is acquisition of knowledge of the attribute structure of the domain. Experts know more than simply what category an object belongs to; they understand the attribute relationships that obtain within the domain (e.g., for carpenter's tools, objects which are to be used for banging must have heads made of hard material). The fourth aspect is progress from novice to expert in a domain. The fifth aspect is the use of operating principles for early conceptual development. Examination of the use of particular operating principles provides a direct test of the proposed theory. To accomplish these objectives, 12 studies are proposed. The children who will participate in these studies are between ages 2 years and 6 years. the first 11 studies are cross-sectional and involve either experimental or observational techniques. Each is designed to answer specific question about one or more of the aspects of expertise described above. The last study is short-term longitudinal. In this study, a specially designed board game will be used to examine the development of domain-specific expertise, from novice to beginning expert. The results of these studies will provide a solid basis for testing, refining, and expanding the scope of the Principal Investigator's theory of early conceptual development. Importantly, the results also will provide a framework for delineating guidelines for intervention programs designed to facilitate language and conceptual development.